It is well known that substrates may be provided with several superimposed coating layers in order to obtain coatings with a good decorative effect and, at the same time, good protective properties. For example, multilayer coats of lacquer are applied preferably according to the so-called "base coat/clear coat" method. After a short exposure to air without a stoving step, the initially applied pigmented base lacquer is painted over by the "wet-on-wet" method with a clear lacquer. Both layers are then stoved together. Metal-effect lacquers are preferably processed in the automobile industry by this method. Because of the composition of the base lacquer, high proportions of solvents are consumed in this procedure. The problem therefore existed of converting such a lacquer system to a water-dilutable basis.
Such a vehicle must
be applicable trouble-free by electrostatic spraying with, for example, high rotation bells PA1 be oversprayable "wet-on-wet" with conventional or water-dilutable clear lacquers, that is, in spite of the shortest possible air exposure times and/or the shortest possible heating times, no interfering incipient solution phenomena should be seen through the (transparent) covering layer; PA1 orient and fix metal-effect pigments spatially in order to achieve a good optical effect; PA1 for repair purposes, lead to films with excellent mechanical properties already after being cured at low temperatures, such as 80.degree. C.; together with the clear lacquer, these films should be highly resistant to weathering. PA1 a) 0.65 to 9 parts by weight of carboxyl group-free unsaturated monomers which, aside from the unsaturated bond, do not contain any other groups, which are reactive under the polymerization and curing conditions, together with 0 to 65% by weight, based on the total monomers, of polymerizable, hydroxyl group-containing monomers and/or 0 to 7% by weight, based on the total monomers, of ethylenically unsaturated monomers in PA1 b) 1 part by weight, based on the resin portion, of an aqueous dispersion of a urea group-containing polyurethane, which was produced by chain extension of an NCO group-containing prepolymer based on polyester with an NCO group content of 1.0 to 10% with at least two urethane groups per molecule, with carboxyl groups corresponding to an acid number of 20 to 50 and a number average molecular weight of 600 to 6,000, with a polyamine with primary and/or secondary amino groups and/or with hydrazine in an aqueous medium without addition of emulsifier, PA1 n=1 to 6 and preferably n=1, R"=--R' or --COOC.sub.n H.sub.2n+1 ; R"'=a linear or branched C.sub.1-6 alkyl group with 1 to 3 OH groups, and X=--COO--, --CONH--, --CH.sub.2 O-- or --O--. PA1 a) 0 to 12% by weight and especially 1 to 10% by weight of .alpha.,.beta.-unsaturated carboxylic acids, PA1 b) 10 to 65% by weight and especially 15 to 50% by weight copolymerizable hydroxyl group-containing monomers, especially ethylenically monounsaturated hydroxyl group-containing monomers, PA1 c) 0 to 7% by weight and especially 0.1 to 5% by weight and more particularly 1.0 to 3% by weight of polyunsaturated monomers, especially ethylenically polyunsaturated monomers and PA1 d) 16 to 90% by weight and especially 35 to 85% by weight of unsaturated, especially ethylenically unsaturated monomers, which do not carry any reactive groups other than the unsaturated bond. PA1 a) of an hydroxyl group-containing saturated polyester, obtained by the condensation of one or more multifunctional alcohols and aromatic, aliphatic and/or cycloaliphatic polycarboxylic acids or their anhydrides, with PA1 b) one or more aromatic, cycloaliphatic or aliphatic polyisocyanates, with the proviso that the equivalent ratio of OH groups to isocyanate groups is 10:1 to 1:1 and a portion of the multifunctional alcohols additionally contains a free carboxyl group, which has two additional aklylol groups as substituents at the a-carbon atoms, such as dimethylolpropionic acid. The proportion of free carboxyl groups is such, that the acid number of the resin is 40 to 100. The preparation of such resins, which are water soluble after neutralization because of their acid number of 40 to 100, is described, for example, in the German Offenlegungsschrift 1,915,800.
In the development of such water-dilutable systems, problems arise, which are difficult to solve and are attributable to the special properties of water and to the low heating temperatures, at which practically no cross linking reactions take place. Attempts have been made in the past to arrive at suitable water-dilutable systems. However, it was not possible to combine all the desired properties optimally in one vehicle.
In the German Offenlegungsschrift 2,860,661, water-dilutable vehicles based on acrylate are described, which contain polymer microparticles, which are insoluble in nonaqueous solvents and water. They are prepared with the help of steric dispersion stabilizers by polymerization in nonaqueous solvents and subsequently transferred into the aqueous medium. This conversion process is very costly and also very susceptible to failure, because the effectiveness of the dispersion stabilizer can be affected by variations in the synthesis process. In the European Patent A 38 127, these vehicles are used for the preparation of metallic base coats, the aluminum platelets or pigments being brought into the lacquer with the help of a melamine resin. At low stoving temperatures, this resin acts like a plasticizer and worsens the moisture susceptibility of the film.
In the German Patent 2,736,542, primers for metal spools are described, which are based on acrylate resin/polyurethane dispersions. Combinations of polyurethane dispersions with latex polymers are used here, optionally with addition of melamine resins. Special combinations, which are matched to the requirements of metallic base coats, are not named.
In the German Offenlegungsschrift 1,953,348, a free radical emulsion polymerization of vinyl monomers is carried out in the presence of aqueous dispersions of high molecular weight polyurethanes with anionic groups. In the examples, the polyurethane dispersions contain alkali salts of 1:1 adducts of diamines and sultones. Because of the non-volatile salt content in the film, adhesion problems arise in multilayer lacquer structure. There are no special references to the selection of suitable polyurethane resins or of unsaturated monomers, with which the requirements of metallic base coats are fulfilled.
In the German Offenlegungsschrift 2,363,307, vinyl monomers are reacted by free radical polymerization in the presence of a polyurethane latex. In the examples, only polyurethane dispersions based on polypropylene glycol are used, which moreover are reacted with a relatively low monomer content. In so doing, films are formed in multilayer structures, which are not sufficiently resistant to the effects of moisture. Here also, special selection conditions cannot be inferred for the preparation of metallic base coats.
The German Offenlegungsschrift 3,545,618 relates strictly to polyurethane dispersions for the preparations of water base coats for multilayer coatings. There is no reaction with unsaturated monomers. They are prepared by reaction of linear polyether and polyester glycols, to which triols have been added, with diisocyanates in inert organic solvents. The resulting polyurethanes must be readily soluble in the solvents used. Neutralization and dilution with water is technically controllable only if relatively low molecular weight resins and increased amounts of solvent are used. The film properties show a high solvent sensitivity, which leads to difficulties during the application of the clear lacquer on the base coat. Higher molecular weight polyurethanes can be emulsified only with difficulty in water and lead to emulsions with very coarse particles. Stable systems require a high salt group content.
In the German Offenlegungsschrift 3,210,051, water-dilutable polyurethane dispersions are used for the preparation of metallic base coats. The dispersions are obtained by dispersing a neutralized anionic NCO prepolymer in water and subsequently subjecting it a chain extension with polyamines. These coating materials create problems when used under practical conditions in batch lacquering processes, because the rapidly drying coating materials in the application devices used (for example, lacquer spray gun or electrostatically supported high rotation bells) coagulate at the walls and adhere there so well, that they can be removed again only with great difficulty. The danger of forming pin-holes and gel particles in the film surface is therefore very great and a quick shade change is made formidable by difficult cleaning operations. Additional large amounts of solvent are introduced into the mixture by the addition of acrylate resin solutions. The addition of melamine resins makes it difficult to produce suitable water-resistant films at low stoving temperatures, as required for repairs.
In U.S. Pat. No. 4,318,833, thermoplastic polymers are prepared by polymerization of unsaturated monomers in the presence of fully reacted, water-soluble polyurethanes. The polymers either have oxidative drying properties or, after addition of cross linking agents such as aminoplasts, phenoplasts or blocked polyisocyanates, can be stoved during 10 to 15 minutes at 125.degree. to 175.degree. C. as coating lacquers, which can be pigmented. The water-soluble polyurethane resin can be prepared from polyether or polyester glycols and contains anionic or cationic groups. The resin must be readily soluble in the inert solvents required for the preparation. The neutralization and dilution with water can be controlled on an industrial scale only if relatively low molecular weight resins and increased amounts of solvent are used. The examples of this US patent show only that a polymerization of unsaturated monomers in the presence of a polyurethane dispersion is possible in the ratio of 2 parts of polyurethane to 1 part of monomers (based on the solids content). It has been ascertained that, with an increased monomer content, polymers are obtained, which are difficult to filter and lead to pinhole-containing lacquer films with an unusable surface. In some cases, the films show insufficient resistance to the action of water and solvents.